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Clocks & Timers
Clocks & Timers Forum
LMH2180 75 MHz Dual Clock Buffer
I am looking for the time delay between Enable on and output active, and between Enable off and output to hi impedance.
Can somebody please help me to get this info? Thanks
Measurement on a typical LMH2180 unit in the Lab shows: - Turn-on time from EN Low-High to Output active = ~650ns - Turn-off time from EN High-Low to Output Hi-Z = ~500ns
Please let us know if you require any further assistance.
Thank you very much for your clear answer. Unfortunately these time are far to long
for my application: switching TCXO outputs (25MHz, clipped sine, 0.8Vpp) with switching
times in the order of a few nsec!! Do yopu have a suggestion for an alternative?
If you require an dual channel analog clock buffer to preserve the clipped sine signal, you could use a dual op amp with low-noise and disable feature in unity-gain configuration. One possible option is OPA2836 which has a disable time of 25 ns and enable time of 200 ns (typical) at Vs = 2.7 V.
If you are OK with converting the clipped sine wave to a square wave, the LMK00105 (5 outputs) is a LVCMOS buffer with Hi-Z capability, 1.5 to 3.3V adjustable output supply, and synchronous output enable (OE). The OE control input is synchronized to the input clock to ensure runt-pulse-free operation when the OE toggled H to L (disabled) or L to H(enabled). The enable/disable time is 2 input clock cycles typical (3 cycles max). So with a 25 MHz input clock, the typical enable/disable time will be between 80 to 120 ns.
Thanks for your thorough reply. I do indeed need to preserve the clipped sine property, the processor chip which needs to be clocked requires this. I need to switch from one clock to another. I can't however miss any clock cycle, that's why I am looking for devices with enable/ disable times in the order of a few nanoseconds. I may have found CMOS switches which do have this property. Do you have alternative suggestions?
A high-speed analog switch/multiplexer could provide the fast switching you're looking for, but make sure that the off-isolation and crosstalk performance is sufficient for your application especially if the two input clocks you are switching are at different frequencies and/or non-synchronous.
TI has a wide portfolio of these parts, which can be found through the links below:
Analog Switch (Vs <= 5V) Product Tree: http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?familyId=614&family=logic&uiTemplateId=SZVI_TAnalog Switch (Vs > 5V) Product Tree: http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?familyId=1510&family=logic&uiTemplateId=SZVI_TAnalog Switch Guide: http://www.ti.com/lit/sg/slyb125c/slyb125c.pdf
Yes, I guess that's the way to go. I found one switch with on/ off times in the order of 5nsec, that should do the trick. And thanks for your remarks about cross-talk etc.
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